Patent Application
20070254102
In the method for producing SiOx (x<1) of the present invention, a mixture of silicon oxide or silicon dioxide powder and a powder of a material which reduces the silicon oxide or silicon dioxide powder is used for the starting material used for generating silicon oxide gas. Examples of such reducing powder include metal silicon compounds and carbon-containing powders, and among these, use of the one containing a metal silicon powder is preferable in view of (1) increasing the reactivity, and (2) increasing the yield.
The metal silicon powder and the silicon dioxide powder may be mixed at an adequately selected ratio. However, they may be mixed at a mixing ratio of 1<metal silicon powder/silicon dioxide powder<1.1, and in particular, 1.01≦metal silicon powder/silicon dioxide powder≦1.08 in consideration of the oxygen on the surface of the metal silicon powder and the presence of a minute amount of oxygen in the reaction furnace.
In the present invention, the starting material which generates the silicon oxide gas as described above is heated and maintained at a temperature of 1,100 to 1,600° C., and preferably 1,200 to 1,500° C. to thereby generate silicon oxide gas. When the reaction temperature is lower than 1,100° C., the reaction will not sufficiently proceed to detract from the productivity, whereas the reaction at a temperature in excess of 1,600° C. results in the melting of the starting powder mixture, and hence, in the decrease of reactivity and difficulty of selecting the furnace material.
This reaction is conducted by using an inert gas for the furnace atmosphere or at a reduced pressure. From the thermodynamic point of view, a higher reactivity can be attained by the use of a reduced pressure, and the reaction can also be carried out at a lower temperature. Accordingly, the reaction is generally carried out at a pressure of 1 to 100 Pa, and in particular, at 5 to 100 Pa.
In the meanwhile, the metal silicon is heated and retained at a temperature of 1,800 to 2,400° C., and preferably 2,000 to 2,300° C. to generate the metal silicon gas. When the reaction temperature is less than 1,800° C., generation of the metal silicon gas is insufficient and production of the intended SiOx (x<1) will be difficult. Heating to a temperature in excess of 2,400° C. or higher is associated with difficulty in selecting the furnace material.
This reaction is conducted by using an inert gas for the furnace atmosphere or at a reduced pressure. From the thermodynamic point of view, a higher reactivity can be attained by the use of a reduced pressure, and the reaction can also be carried out at a lower temperature. Accordingly, the reaction is generally carried out at a pressure of 1 to 100 Pa, and in particular, at 5 to 100 Pa.
The value of x in the SiOx of the present invention is x<1, and this value can be controlled by the temperature to which the starting material generating the silicon oxide gas and the metal silicon are heated, namely by the vapor pressure of these components, and the weight of the starting material generating the silicon oxide gas and the metal silicon. In the present invention, the value of x in the SiOx is preferably in the range of 0.3<x<0.9, more preferably 0.4≦x≦0.8, and most preferably 0.4<x<0.8. When the value of x is 0.3 or less, the cell may become deteriorated with repeated cycles of use while cell capacity and initial irreversible capacity are reduced. On the other hand, when the value of x is 0.9 or higher, the capacity of the level required by the market may not be satisfied.
When the value of x in the SiOx is within such range, the value of x can be controlled by the vapor pressure and the amount of each of the starting material generating the silicon oxide gas and the metal silicon. More specifically, when the vapor pressure of the starting material generating the silicon oxide gas and the metal silicon is the same and these components are used at the same amount, the value of x is theoretically 0.5. The vapor pressure of the starting material generating the silicon oxide gas and the metal silicon is determined by the temperature to which they are respectively heated, and for example, the vapor pressure of the starting material generating the silicon oxide gas heated to a temperature of 1350° C. and the vapor pressure of the metal silicon heated to a temperature of 2200° C. are at the same level, namely, at 500 Pa.
The starting material generating the silicon oxide gas and the metal silicon may be mixed at an adequate mixing ratio depending on the desired value of x. However, since Si is heated to a higher temperature, a higher efficiency can be realized when Si is used at a higher mixing ratio. More specifically, the starting material generating the silicon oxide gas and the metal silicon may be mixed at a weight ratio of the starting material generating the silicon oxide gas/the metal silicon of 1/5 to 1/1, and in particular, 1/3 to 1/1.5.
The production system used in the production of the present invention is not particularly limited, and the production system may be the one having the mechanism of generating the silicon oxide gas and the mechanism of generating the metal silicon gas separately, or the one having such mechanisms in the same apparatus. Also, the method used in mixing the generated silicon oxide gas and the metal silicon gas is not particularly limited.
The type of the substrate used for the precipitation of the gas mixture is not particularly limited. The preferred, however, is a metal having a high melting point such as SUS and tungsten.
The conditions used in precipitating the gas mixture on the substrate are not particularly limited, and the precipitation can be accomplished by statically placing the substrate on the gas flow passage. However, homogeneous precipitation of the gas mixture on the substrate may be facilitated by constituting the substrate from a relatable body, or by passing a cooling medium such as water through the substrate.
The SiOx (x<1) precipitated on the substrate may be collected by an adequate means such as scraping. The collected SiOx may be pulverized by an adequate means to the desired particle size.
The SiOx (x<1) produced in the present invention may be used for the anode material in producing a lithium ion secondary battery.
The lithium ion secondary battery produced in the present invention has the characteristic feature that it is produced by using the material as described above for the anode active material, and other factors of the battery, for example, the material used for the cathode, anode, electrolyte, separator, and the like and the shape of the battery are not particularly limited. For example, exemplary materials which may be used for the cathode active material include transition metal oxides such as LiCoO2, LiNiO2, LiMn2O4, V2O6, MnO2, TiS2, and MoS2 and chalcogen compounds. Exemplary electrolytes include non-aqueous solution containing a lithium salt such as lithium perchlorate, and exemplary non-aqueous solvents include propylene carbonate, ethylene carbonate, dimethoxy ethane, γ-butyrolactone, and 2-methyl tetrahydrofuran, which may be used alone or in combination of two or more. Various other non-aqueous electrolytes and solid electrolytes may also be used.
The SiOx powder of the present invention may be used by adding an electroconductive agent such as graphite, and the electroconductive agent used is not particularly limited as long as it is an electron-conductive material which does not experience decomposition or deformation in the resulting battery. Exemplary such electroconductive agents include powders or fibers of a metal such as Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, or Si, various forms of graphite such as natural graphite, artificial graphite, various coke breezes, mesophase carbon, gas phase-grown carbon fibers, pitch carbon fibers, PAN carbon fibers, and various sintered resins.
Next, the present invention is described in detail by referring to Examples of the present invention and Comparative Examples, which by no means limit the scope of the present invention. In the following Examples and Comparative Examples, the average particle size is the cumulative weight average (D50) calculated in the measurement of particle size distribution by laser diffractometry.
The SiOx was produced by using the production system shown in
Next, 30 g of this intermediate was pulverized by wet pulverization in a 2 L alumina ball mill using 1,000 g of alumina balls having a diameter of 5 mm for the medium and 500 g of hexane for the solvent under the rotation condition of 1 rpm. The pulverized SiOx powder had the physical properties as shown in Table 1.
Next, a battery was produced by using such SiOx powder for the anode active material, and this battery was evaluated by the procedure as described below.
First, 45% by weight of an artificial graphite having an average particle diameter of 5 μm and 10% by weight of polyvinylidene fluoride were added to the SiOx powder produced as described above, and after adding N-methyl pyrrolidone to produce a slurry, this slurry was applied to a copper foil having a thickness of 20 μm. After drying at 120° C. for 1 hour, an electrode was pressed at an elevated pressure in a roller press, and an anode of 2 cm2 was finally punched from this sheet.
In order to evaluate the charge and discharge characteristics of the resulting anode, a secondary battery for evaluation purpose was prepared by using a lithium foil for the counter electrode, a non-aqueous electrolyte solution of lithium hexafluorophosphate in a 1/1 (volume ratio) mixture of ethylene carbonate and dimethyl carbonate for the non-aqueous electrolyte, and a polyethylene microporous film having a thickness of 30 μm for the separator.
The thus produced lithium ion secondary battery was allowed to stand overnight at room temperature, and, placed in a secondary battery charge and discharge tester manufactured by Nagano. The test cell was charged at a constant current of 1 mA until the voltage of the test cell reached 0 V, and after reaching 0 V, the cell was charged by reducing the current so that the cell voltage would remain at 0 V. The charging was terminated when the current value reduced to less than 20 μA. The discharge was conducted at a constant current of 1 mA, and the discharge was terminated when the cell volgate exceeded 1.8 V, and discharge capacity was measured.
The lithium ion secondary battery prepared as described above for evaluation was evaluated for its discharge capacity after 10 cycles of such charge and discharge cycles. The results are shown in Table 1.
Japanese Patent Application No. 2006-121954 is incorporated herein by reference.
Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-121954 | Apr 2006 | JP | national |
